@ArnaudH ,
I cannot see your data, so am only guessing here - as an example if you had the field LinkA in List A and LinkB in List B and they had the same value (a "key" field if you like) and the changed field was (to use my previous example) is called Status. If there was only one record in List B matching the record you want to colour in List A, then this is easy
If(
LookUp(
'List B'
LinkB = ThisItem.LinkA
).Status = "Completed",
Color.Green,
Color.Grey
)
If there are multiple matches in list A to the changed item in List B, then all those items in the gallery will turn green. There has to be a one-to-one relationship for this to work.
Please click Accept as solution if my post helped you solve your issue. This will help others find it more readily. It also closes the item. If the content was useful in other ways, please consider giving it Thumbs Up.
MVP (Business Applications) Visit my blog Practical Power Apps